1. Field of the Invention
The present invention relates to a two speed pulley assembly. More specifically, the present invention relates to a two speed pulley assembly, controllable to provide two different drive speeds to an automotive accessory (such as an alternator) or industrial mechanism, having a novel traction drive mechanism.
2. Description of the Prior Art
Typically, a pulley used with an automotive accessory is of a one-piece design and is capable of only one possible drive speed ratio. A belt driven by the crank shaft of the engine drives the pulley. The belt provides a single drive speed which is based on the ratio of effective diameters of the driving (crankshaft) pulley and driven (alternator) pulley: EQU N.sub.alt =N.sub.crank .times.(D.sub.crank /D.sub.alt)
where:
N.sub.alt =alternator speed PA1 N.sub.crank =crankshaft speed PA1 D.sub.crank =crankshaft pulley diameter PA1 D.sub.alt =alternator pulley diameter
Concerns with single speed systems arise when the engine and crankshaft are running at low or idle speeds. At such speeds, inadequate electrical output from the alternator can result, especially with the additional electrical loads (front and rear defrosters, electric seats, mirrors and windows, high intensity headlamps, etc. all working simultaneously) found in today's luxury cars. The current solution to this low alternator output has been to reduce the alternator pulley diameter. By reducing the alternator pulley diameter, the alternator is driven at a higher ratio and ultimately a higher top end speed. While concerns of inadequate output at low speeds are eliminated, excessive heat and noise are generated as are excessive wear and tear on the alternator bearings. While low electrical output conditions are only occasionally experienced, the problem is sufficient enough to prompt automotive companies into using more sophisticated pulleys and complex drive systems, with their associated higher cost, in an attempt to eliminate or further reduce the problem of low electrical output from the alternator.
Two speed systems that involve the use of two coupled alternator pulleys and two drive belts have been developed. Such systems typically feature a standard serpentine accessory drive belt and a second, clutchable, drive belt driven off of a second pulley, typically on one of the accessories (i.e. water pump). During normal operation, an electromagnetic clutch is disengaged, allowing the standard belt to drive the alternator through one of two coupled pulleys. In most cases, the alternator speed attained under this circumstance is somewhat lower than the corresponding speed of a single speed alternator. This is because the diameter of that portion of the pulley driven by the standard belt is usually larger than the diameter of a standard, single belt pulley. This diameter change lowers peak alternator speeds.
At low engine speeds, an electromagnetic clutch is actuated to drive the second drive pulley and belt. The belt in turn drives the alternator's second pulley, located outboard or axially further from the alternator itself. This second alternator pulley has a diameter which is smaller than the first pulley. By virtue of this smaller diameter, the outboard alternator pulley spins faster, overdriving the larger first pulley via a one-way clutch. This provides a higher alternator drive ratio.
By choosing the appropriate diameters for the crank pulley, second drive pulley, and the coupled alternator pulleys, a suitable higher alternator speed can be achieved, one which provides greater electrical output at low engine speeds. Unfortunately, such a system is expensive in that it requires not only an electromagnetic clutch, but also a second belt, a second tensioner, a second drive pulley and the two coupled driven pulleys. For certain applications, the added expense will prove prohibitive.
Another limitation with the above two-belt type of system is that the coupled driven pulleys form an assembly having an axial length that may not fit in existing engine compartments and may require expensive and complicated packaging changes within the engine compartment. Because of this length increase and the additional belt load on the outboard pulley, the moment load on the front alternator bearing increases, typically requiring a larger bearing to be specified in such applications. Such a bearing change would also require significant design changes to the alternator itself, increasing cost and preventing commonality between parts.
In view of the foregoing limitations and disadvantages of the prior art devices, as well as other disadvantages not specifically mentioned above, it should be apparent that there still exists a need in the art for an improved pulley system which offers better electrical output from the alternator at low engine speeds and reduced peak alternator speeds at high engine speeds, while minimizing overall packaging requirements and system costs.
It is therefore a primary object of this invention to fulfill that need by providing a two speed pulley assembly capable of sufficiently driving an automobile accessory, such as an alternator, at low engine speeds.
It is a further object of the present invention to provide a two speed pulley assembly having minimal packaging requirements enabling it to be positioned within the typical space requirements of a standard, single speed pulley.
Another object of the present invention is to reduce the moment load on the front alternator bearing, compared with a two-belt system, to allow existing alternators to be used.
It is another object of the present invention to provide a two speed pulley assembly that is less expensive to produce than current two belt pulley systems.
Still another object of the present invention to provide a two speed pulley assembly that reduces wear and tear on the overall system by limiting the output speed of the pulley assembly at high engine speeds.